Upon saving main information such as documents, images, and the like, sub information as auxiliary information associated with the main information is often saved together with it. The sub information includes, for example, time information indicating the generation time of the main information. More specifically, when main information is a document, the generation time of that document is used; when main information is an image, the photographing time of that image is used.
Upon saving main information of a specific type for a long period of time, the time information as the sub information is important. This is because a method of displaying a list by time-serially sorting a plurality of pieces of main information, comparing a plurality of pieces of main information at the same time, and searching for desired main information based on time information is effective.
However, upon managing time information, time differences between countries or areas pose a problem. In ordinary life, local time is used but it is not internationally common. Furthermore, in a country or area that adopts the Daylight Saving Time (D.S.T.) system, the time differences among the international standard time, local time, and D.S.T. time differ depending on seasons.
For this reason, conventionally, as sub information the local time and information indicating if the local time is the D.S.T. time are saved together with main information, the local time or international standard time and information of a three-letter code indicating an area (city or the like) are stored together with main information, or the international standard time alone is simply saved together with main information. Conventionally, when main information is used, the sub information is converted into an international standard time or local time on the basis of such sub information and time zone information of a season specified by the sub information.
However, in order to manage time zone information used to convert sub information of every seasons into an international standard time or local time, considerable cost is required, and such organization, institution, or the like must be established.
Respective areas adopt various time operation systems. For example, a time system (e.g., D.S.T.) different from other years may be adopted in only a year in which the Olympic games are held, or D.S.T. may be adopted through several years irrespective of seasons due to political reasons during a war. Or countries or areas may be remapped due to revolution, independence, reform of laws, and the like, and a system that defines the local time may change accordingly. That is, the system which defines the local time may change for various reasons even in a single area, and even when the local time and area are specified, the local time cannot often be converted into an international standard time unless the time system of that season is considered. The same applies to a case wherein an international standard time is converted into the local time of a given area on the basis of the international standard time and area.
For example, upon sequentially extracting images that indicate count-down scenes of new years from a huge number of stored images, an image search is required on both the international standard times and local times. To meet such requirement, only time zone information of the photographing time is used.
As described above, according to the conventional method, time zone information used to convert sub information into an international standard time or local time must be permanently managed, and it is difficult to achieve consistent time management.